1. Field of Invention
The present invention relates to a method and apparatus for increasing the etch resistance of photoresists which are suitable for use in the production of electronic devices such as integrated circuits. More particularly, the invention provides an improved process for increasing the etch resistance of positive working chemically amplified photoresists such as 193 nm, 157 nm, deep-UV 248 nm and x-ray wavelength sensitive, and electron beam sensitive photoresists while improving and maintaining fidelity of lithographic features and critical dimensions.
2. Description of the Related Art
The production of positive photoresists is well known in the art as exemplified by U.S. Pat. Nos. 3,666,473; 4,115,128 and 4,173,470. These contain aqueous alkali soluble polyvinyl phenol or phenol formaldehyde novolak resins together with light sensitive materials, usually a substituted naphthoquinone diazide compound. The resins and sensitizers are dissolved in an organic solvent and are applied as a thin film coating to a substrate suitable for the particular application desired. The resin component of photoresist formulations is soluble in an aqueous alkaline solution, but the photosensitizer is not. Upon imagewise exposure of the coated substrate to actinic radiation, the exposed areas of the coating are rendered more soluble than the unexposed areas. This difference in solubility rates causes the exposed areas of the photoresist coating to be dissolved when the substrate is immersed in an alkaline developing solution, while the unexposed areas are substantially unaffected, thus producing a positive image on the substrate. The uncovered substrate is thereafter subjected to an etching process. Frequently, this involves a plasma etching against which the resist coating must be sufficiently stable. The photoresist coating protects the covered areas of the substrate from the etchant and thus the etchant is only able to etch the uncovered areas of the substrate. Thus, a pattern can be created on the substrate which corresponds to the pattern of the mask or template that was used to create selective exposure patterns on the coated substrate prior to development.
Photoresists are either positive working or negative working. In a negative working resist composition, the imagewise light struck areas harden and form the image areas of the resist after removal of the unexposed areas with a developer. In a positive working resist the exposed areas are the non-image areas. The light struck parts are rendered soluble in aqueous alkali developers. The ability to reproduce very small dimensions, is extremely important in the production of large scale integrated circuits on silicon chips and similar components. As the integration degree of semiconductor devices becomes higher, finer photoresist film patterns are required. One way to increase circuit density on a semiconductor chip is by increasing the resolution capabilities of the resist. Positive photoresists have been found to be capable of much higher resolution and have almost universally replaced negative resists for this purpose.
The optimally obtainable microlithographic resolution is essentially determined by the radiation wavelengths used for the selective irradiation. However the resolution capacity that can be obtained with conventional deep UV microlithography has its limits. In order to be able to sufficiently resolve optically small structural elements, wavelengths shorter than deep UV radiation must be utilized. The use of UV radiation has been employed for many applications, particularly radiation with a wavelength of 157 nm, 193 nm and 248 nm. In particular, the radiation of lasers is useful for this purpose. The deep UV photoresist materials that are used today, however, are not suitable for 157 nm, 193 nm and 248 nm exposure. Materials based on phenolic resins as a binding agent, particularly novolak resins or polyhydroxystyrene derivatives have too high an absorption at wavelengths and one cannot image through films of the necessary thickness. This high absorption results in side walls of the developed resist structures which do not form the desired vertical profiles. Rather they have an oblique angle with the substrate which causes poor optical resolution characteristics at these short wavelengths. Polyhydroxystyrene based resists can be used in top surface imaging applications in which a very thin (xcx9c500 xc3x85) layer of resist is required to be transparent at ArF laser exposure wavelengths.
Chemical amplification photoresists have been developed, which have been found to have superior resolution. 157 nm, 193 nm and 248 nm photoresists are based on chemically amplified deprotection. With this mechanism, a molecule of photogenerated acid catalyzes the breaking of bonds in a protecting group of a polymer. During the deprotecting process, another molecule of the same acid is created as a byproduct, and continues the acid-catalytic deprotection cycle. The chemistry of a 157 nm, 193 nm and 248 nm photoresist is based on polymers such as, but not limited to, acrylates, cyclic olefins with alicyclic groups, and hybrids of the aforementioned polymers which lack aromatic rings. However, chemically amplified resist films have not played a significant role in the fine pattern process using deep UV because they lack sufficient etch resistance, thermal stability, post exposure delay stability and processing latitude. While such photoresists are sufficiently transparent for deep uv radiation, they do not have the etching stability customary for resists based on phenolic resins for plasma etching. A typical chemical amplification photoresist film comprises a polymer, a photoacid generator, and other optional additives. The polymer is required to be soluble in the chosen developer solution, and have high thermal stability and low absorbance to the exposure wavelength in addition to having excellent etch resistance. It would be desirable to overcome the above mentioned problems and to provide a photoresist film superior in etch resistance, as well as transmittance to deep UV.
There have been several attempts to solve this problem. One attempt to improve the etching stability of photoresists based on meth(acrylate) polymers introduced cycloaliphatic groups into the meth(acrylate) polymers. This leads to an improvement in etching stability, but not to the desired extent. Another proposal aims at producing sufficient etching stability only after irradiation in the resist coating. It has also been proposed to treat the substrate with the finished, developed, image-structured photoresist coating with specific alkyl compounds of magnesium or aluminum, in order to introduce the given metals in the resist material as etching barriers (See U.S. Pat. No. 4,690,838). The use of metal-containing reagents, however, is generally not desired in microlithography process, due to the danger associated with contamination of the substrate with metal ions.
U.S. Pat. No. 6,319,655, which is incorporated herein by reference, describes a process for improving the etch resistance of chemically amplified resists, in particular 193 nm sensitive photoresists, using a large area electron beam exposure. Electron beam exposure of chemically amplified photoresists, in particular 193 nm sensitive photoresists has been shown to improve the etch resistance and thermal stability of these photoresists. Many different formulations of chemically amplified photoresist utilized for 193 nm exposure have been developed. Some examples of materials used for 193 nm lithography are given in U.S. Pat. No. 6,319,655. For the next generation of lithography, new resist materials sensitive to 157 nm irradiation will be utilized for this application. Some of these materials (incorporated herein by reference) are listed in xe2x80x9cOrganic Imaging Materials, A View of the Futurexe2x80x9d by Michael Stewart et al., Journal of Physical Organic Chemistry, J. Phys. Org. Chem. 2000; 13: 767-774, xe2x80x9c157 nm Resist Materials: Progress Reportxe2x80x9d by Colin Brodsky et al., J. Vac. Sci. Technol. B 18(6), November/December 2000, 3396-3401, and in xe2x80x9cSynthesis of Siloxanes and Silsesquioxanes for 157m Microlithographyxe2x80x9d by Hoang V. Tran, et al, Polymeric Materials: Science and Engineering 2001, 84. Due to the volatility of the additives in these resist materials, electron beam exposure causes the expulsion of these additives, which causes shrinking of the resist. For 248 nm, 193 nm, and 157 nm resist technologies, chemical amplification is used to achieve high photo speed at the low exposure energies of the selected wavelength sources. For future lithography generations using extreme ultraviolet and x-ray wavelengths of 1-100 nanometers, similar chemically amplified resists are anticipated. The basic resist design concept is to start with a resin that has good transparency at the selected wavelength. The resin must be highly soluble in aqueous base developer chemistry. To make the resin insoluble in the developer, dissolution inhibitors, sometimes called blocking or protecting groups, are attached to the resin. These are usually very large, or bulky, molecules that are attached to the resin via bonds that can be easily cleaved. In most advanced resist systems there are usually several types of molecules attached to the resin in addition to the dissolution inhibitor. These include molecules that enhance the etch resistance of the material as well as molecules that help with lithographic performance. All of these molecules are attached to the resin via a link that is easily cleaved. The chemical amplification is achieved by adding a small amount of a photo-acid generator (PAG). This is a compound that generates a proton (H+) when exposed at the appropriate wavelength. These are usually onium salts, such as sulfonium salts, but it can be any of a number of suitable compounds. When the PAG is exposed and the proton is generated, the proton cleaves the nearest available bond between the resin and dissolution inhibitor. This cleaving reaction generates another proton, which cleaves the next nearest bond, and so on. This reaction can occur during the optical exposure, for low activation energy resists, or during the subsequent post-exposure bake (PEB), for high activation energy resists. The result of the de-protection reaction is the formation of an acid, which is then soluble in an aqueous base developer. As a result of the cleaving of the link between the resin and the blocking group, the blocking molecule usually leaves the resist as a volatile. In this way the resist can be fully exposed even though the incident optical exposure dose is very low, on the order of 10 to 20 mJ/cm2. After the optical exposure and completion of the de-protection reactions, the resist in the exposed areas can shrink from ten to twenty percent. This is due to the loss of the bulky protecting groups as volatiles. This reaction does not happen in the unexposed areas, which provides the contrast to form the images. Since the unexposed resist still contains the resin with the attached blocking groups, it is susceptible to shrinkage if these molecules are removed.
Due to the constraints of the resist design, and since the blocking groups are easily cleavable, the blocking groups can be removed by other means. One reaction path is thermal activation, where the resist is heated to a temperature that thermally breaks the bonds. This happens at different temperatures for the different protection groups but can be a low as 40xc2x0 C. to as high as 200xc2x0 C. Thermal activation results in the loss of the blocking groups as volatiles and a shrinkage of the resist. The blocking groups can also be removed by other radiation sources including plasma discharges, or accelerated particles.
During electron beam exposure, a reaction that is similar to the optical exposure can occur which cleaves the link between the protecting groups and the resin resulting in shrinkage of the resist. This reaction, and the associated resist shrinkage, is accelerated as the resist is heated by the energy of the incident electron beam. Since the full thickness of the resist is targeted for stabilization, substantial mass loss, and shrinkage, can result from the electron beam exposure. Because the interface between the resist and substrate is constrained, the remainder of the resist shrinks in three dimensions. This leads to a phenomenon know as xe2x80x9cpullbackxe2x80x9d where the top of the resist shrinks relative to the bottom. This effect is most pronounced on lithographic features such as contacts, line ends, and feature corners. The pullback phenomenon has undesired effects on the features, which make them unacceptable for device fabrication. This shrinkage occurs throughout the exposed regions of the photoresist and can cause deformation in the form of pullback on the upper portions of lithography features.
Many attempts have been made to correct or eliminate this resist deformation or pullback by using different process steps with the electron beam exposure. Lower current density exposures have been attempted to minimize the shrinkage as well as surface curing of the photoresist, that is, lowering the energy of electrons such that only the upper portion of the resist receives the electron beam exposure. Higher doses of electrons have been utilized at the lower portion, relatively to the upper portion, of the resist in an effort to minimize this pullback. In addition, different formulations of photoresist have been attempted to minimize the shrinkage and expulsion of resist components to minimize the pullback. Lower flux electrons with longer exposure times have also been utilized to minimize resist heating effects thereby reducing the temperature of the resist during electron beam exposure to no more than 50xc2x0 C. All of these attempts have failed at reducing the pullback effect caused by the electron beam hardening process.
Surprisingly, it has now been found that by simultaneously actively cooling the wafer to hold a temperature below 20xc2x0 C. and preferably below 10xc2x0 C. that the pullback on the upper region of lithographic images in resist can be virtually eliminated during electron beam processing. This unexpected result is due to the fact that the glass transition temperature of these photoresists is much higher than 20xc2x0 C. For example, these photoresists are typically baked at temperatures in excess of 80xc2x0 C. after lithographic patterning. It is therefore surprising since there is very little difference in volume shrinkage and no compositional changes (at least detected by FTIR), that cooling the resist would effect the resist deformation.
Eliminating deformation of the lithographic features during e-beam exposure, the holding of the vertical wall profile, and the improvement in the uniformity and circularity of the contact holes in photoresist is an unexpected result because the resist exhibits almost the same amount of volume shrinkage (measured vertically) as in the known process described in U.S. Pat. No. 6,319,655. There is only a slight change in the total bulk shrinkage between the standard process and the cooled process, and the chemical composition of the film is not altered by cooling. Therefore it has been unexpectedly found that cooling the wafer results in a marked improvement to the stability of the lithographic features of these chemically amplified resists.
The invention provides a method for producing an etch resistant image, which comprises:
(a) coating and drying a photosensitive composition onto a substrate, which photosensitive composition comprises:
(i) at least one water insoluble, acid decomposable polymer which is substantially transparent to ultraviolet or x-ray radiation, wherein said polymer is present in the photosensitive composition in an amount sufficient to form a uniform film of the composition components when it is coated on a substrate and dried;
(ii) at least one photosensitive compound capable of generating an acid upon exposure to sufficient activating ultraviolet, electron beam or x-ray radiation energy, said photosensitive compound being present in an amount sufficient to substantially uniformly photosensitive the photosensitive composition;
(b) imagewise exposing the photosensitive composition to sufficient activating ultraviolet, electron beam or x-ray radiation energy to cause the photosensitive compound to generate sufficient acid to decompose the polymer in the imagewise exposed areas of the photosensitive composition;
(c) developing the photosensitive composition to thereby remove the exposed nonimage areas and leaving the unexposed image areas of the photosensitive composition;
(d) irradiating the image areas of the photosensitive composition to sufficient electron beam radiation to thereby increase the resistance of the photosensitive composition in the image areas to an etchant while simultaneously cooling the photosensitive composition during electron beam radiation to maintain the photosensitive composition at a temperature of less than about 20xc2x0 C.
The invention also provides a method for producing a microelectronic device image, which comprises:
(a) coating and drying a photosensitive composition onto a semiconductor substrate, which photosensitive composition comprises:
(i) at least one water insoluble, acid decomposable polymer which is substantially transparent to ultraviolet or x-ray radiation, wherein said polymer is present in the photosensitive composition in an amount sufficient to form a uniform film of the composition components when it is coated on a substrate and dried;
(ii) at least one photosensitive compound capable of generating an acid upon exposure to sufficient activating ultraviolet, electron beam or x-ray radiation energy, said photosensitive compound being present in an amount sufficient to substantially uniformly photosensitize the photosensitive composition;
(b) imagewise exposing the photosensitive composition to sufficient activating ultraviolet, electron beam or x-ray radiation energy to cause the photosensitive compound to generate sufficient acid to decompose the polymer in the imagewise exposed areas of the photosensitive composition;
(c) developing the photosensitive composition to thereby remove the exposed nonimage areas and leaving the unexposed image areas of the photosensitive composition;
(d) irradiating the image areas of the photosensitive composition to sufficient electron beam radiation to thereby increase the resistance of the photosensitive composition in the image areas to an etchant while simultaneously cooling the photosensitive composition during electron beam radiation to maintain the photosensitive composition at a temperature of less than about 20xc2x0 C.
The invention further provides a microelectronic device image produced by a process, which comprises:
(a) coating and drying a photosensitive composition onto a semiconductor substrate, which photosensitive composition comprises:
(a) coating and drying a photosensitive composition onto a semiconductor substrate, which photosensitive composition comprises:
(i) at least one water insoluble, acid decomposable polymer which is substantially transparent to ultraviolet or x-ray radiation, wherein said polymer is present in the photosensitive composition in an amount sufficient to form a uniform film of the composition components when it is coated on a substrate and dried;
(ii) at least one photosensitive compound capable of generating an acid upon exposure to sufficient activating ultraviolet, electron beam or x-ray radiation energy, said photosensitive compound being present in an amount sufficient to substantially uniformly photosensitive the photosensitive composition;
(b) imagewise exposing the photosensitive composition to sufficient activating ultraviolet, electron beam or x-ray radiation energy to cause the photosensitive compound to generate sufficient acid to decompose the polymer in the imagewise exposed areas of the photosensitive composition;
(c) developing the photosensitive composition to thereby remove the exposed nonimage areas and leaving the unexposed image areas of the photosensitive composition;
(d) irradiating the image areas of the photosensitive composition to sufficient electron beam radiation to thereby increase the resistance of the photosensitive composition in the image areas to an etchant while simultaneously cooling the photosensitive composition during electron beam radiation to maintain the photosensitive composition at a temperature of less than about 20xc2x0 C.
The invention still further provides an electron beam exposure apparatus which comprises
a) an enclosure;
b) an electrostatic chuck within the enclosure for holding a wafer during electron beam exposure, which electrostatic chuck comprises:
i) an electrically conductive wafer support having an electrically conductive top surface;
ii) a nonelectrically conductive layer on the wafer support;
iii) means for applying a substantially uniform electric field across said electrically conductive surface for holding a wafer on the support via the nonelectrically conductive layer;
b) an electron beam source within the enclosure which directs a wide, large beam of uniform electron beam radiation toward the wafer support;
c) a refrigerant within the enclosure which maintains a wafer positioned on the wafer support at a temperature of less than 20xc2x0 C. during electron beam irradiation.